In certain helicopters used for specific applications, such as helicopters based on seagoing ships, it has long been known to fold the main rotor blades to positions adjacent the fuselage of the helicopter during nonuse of the helicopter. This facilitates storage of each helicopter in a relatively small space as well as rendering helicopters which are stored in the open less vulnerable to wind gusts and the like during storage.
In order to fold the blades, it is necessary that each blade assume a predetermined position with respect to the blade fold hinge, and with respect to the fuselage of the aircraft. Therefore, before folding blades, the main rotor is indexed to a predetermined position which puts all of the blades on the rotor in a position where each may be folded to a position alongside the fuselage. Thereafter, the pitch angle of each of the blades is adjusted to a desired position and the pitch angle is locked by means of pins, so that the pitch angle of the blades will not thereafter vary as the blades are being folded.
As is known, the pitch of the main rotor blades of a helicopter is adjusted by push rods which are urged against, and therefore raised and lowered by a swash plate which can tilt varying amounts in any azimuthal direction. It is the tilting of the swash plate which causes the blades to achieve the nominal collective pitch with the desired varying cyclic pitch superposed thereon. As the blades rotate about the main rotor in flight, the push rods connected to the blades and rolling on the swash plate assume various positions in dependence on the tilt of the swash plate and the azimuthal position of the rotor. Thus it is blade motion as the blades rotate which actually achieves the variation pitch in dependence upon the then-current position of the swash plate. Therefore, adjusting the pitch of the rotor blades prior to folding requires positioning of the swash plate, in a fashion similar to that achieved by the pilot controls and/or automatic flight control system during flight.
In the earliest systems, the pitch lock pins were generally displaced by hydraulic pressure, and the pitch positions of the blades were slewed back and forth by operation of manual controls (such as the cyclic pitch stick and the collective pitch stick) until each blade had passed by the pitch lock. The pin was able to snap into place and thereby prevent further pitch change of the blade. However, on-the-fly snap-in of locking pins results in excessive wear. Furthermore, hydraulically actuated lock pins are cumbersome and impede the ability to properly design a rotor head for a helicopter. Electric motor actuated pins, on the other hand, are well suited to rotor head design, but require that the pins be given a sufficient time to engage the blades while they are held in the proper position. This would have required the use of pilot indicators to show the pilot correct blade pitch positions for pin engagement, the pilot moving the controls very slowly to achieve indications and to provide minute adjustments in pitch position once the indicators are lit until pin engagement was achieved. A motorized blade fold lock of a modern type is disclosed in a commonly owned, copending U.S. patent application of Ferris entitled BLADE FOLD RESTRAINT SYSTEM, Ser. No. 35,364, filed on May 2, 1979 and now U.S. Pat. No. 4,284,387.
In order to serve some of the needs of blade folding systems, automatic positioning of the rotor blades in pitch has been achieved as described in a commonly owned, copending U.S. patent application entitled AUTOMATIC LOCK-POSITIONING OF FOLDABLE HELICOPTER BLADES, Ser. No. 195,808, filed on even date herewith by MacLennan and Mulvey. In that system, the positions of the three swash plate servos, for the desired pitch angle of the blades to enable the blades to be locked in pitch prior to folding, is stored in a nonvolatile read/write memory of an automatic flight control system computer. After a first initial locking of the blades, the desired swash plate positions and trim commands (which were required for the automatic flight control system pitch, roll and collective axes) are stored in that memory. In each subsequent folding operation, the final flight control commands and final swash plate servo positions which were used in the preceding blade folding operation are again used to position the blades in pitch before locking. With periodic updating of the ultimate swash plate servo positions which should be achieved in order to engage the pins, and considering the fact that there is some latitute in positioning, due to the shape of the pins and their ability to minutely adjust the pitch as they are engaging, this system can work quite well under many circumstances.
On the other hand, if any of the swash plate servos or position detectors are changed during maintenance between one folding operation and the next, this system will be utilizing obsolete information in a subsequent folding operation. Additionally, it has been found that the storage of digital data relating to both the swash plate servo positions (which provide proper pitch for blade locking) and the automatic flight control system commands (in pitch, roll and collective axes to achieve these positions) requires more nonvolatile read/write storage than is convenient in an airborne flight control system.